1. Field of the Invention
The present invention relates generally to the testing of materials, and more specifically, to an ultrasonic method and apparatus for determining a crack opening load.
2. Description of the Related Art
It is known to test compact tension specimens placed under a load to determine the condition of full crack opening. Referring to FIG. 1, a compact tension specimen 10 has a crack 12 and a notch opening 14 formed at the outer end portion of the crack 12. An extensiometer 16 is placed at the notch opening 14 in order to measure displacement of the notch, and hence, the crack opening. A tension load is applied to the specimen 10 by pulling clevis grips 18 and 20 in opposite directions as indicated by the directional arrows.
Using the apparatus schematically illustrated in FIG. 1, known techniques have been employed for determining crack opening load. The first technique is illustrated in FIGS. 2 and 3 and is known as the "load-reduced displacement" method. In this method, the extensiometer 16 placed at the notch opening 14 measures displacement. By plotting the load on the Y-axis against displacement on the X-axis, determination of reduced displacement leads to a determination of the load at which the crack is actually open. In FIG. 2, "reduced" displacement is determined by the difference between a straight line extrapolation of the upper (straight) data and the measured or actual (curved) data. The extrapolated portion is shown as a broken line and the reduced displacement is indicated by the distance between the broken line and the curved portion of the measured data. The load at which crack opening occurs is illustrated in FIG. 3 as the point at which the reduced displacement does not change with increasing load. As indicated in FIG. 3, this occurs at the vertical portion of the curve.
The second technique is illustrated in FIGS. 4 and 5 and is known as the "load-slope change" method of determining crack opening load. In FIG. 4, load is plotted against displacement as in the load-reduced displacement technique. Changes in slope are measured from regions S1 to S6. Slope increase is plotted in FIG. 5 based on the measured slope changes from FIG. 4. Crack opening is indicated in FIG. 5 where the slope increase changes from perpendicular.
A problem associated with the above-described techniques results from the fact that extensiometers and other similar strain sensors are electrically noisy, and thus, the signals generated by the extensiometers lack the required degree of certainty for precise determination of crack opening load. Another problem is that it is difficult to decide at what point the plot becomes tangent to a line or crosses an axis. The problem is illustrated in FIG. 6 where load is plotted against slope increase. As is evident from FIG. 6, the plotted points of slope increase move to both sides of zero, thereby making determination of an exact crack opening load difficult.
It is generally known to employ acoustic signals to determine the presence of a crack in an object. For example, U.S. Pat. No. 3,911,734 to Mehdizadeh discloses a method of detecting incipient fatigue damage in metal using acoustic emission characteristics obtained during application of a load.
U.S. Pat. No. 4,265,120 to Morris et al. discloses a method of detecting fatigue using acoustic harmonics. A surface acoustic wave is generated at a first position on the object and a harmonic of a generated wave is detected at a second position on the object. The testing method involves relating the characteristics of the detected wave to the remaining useful life of the object.
U.S. Pat. No. 4,522,064 to McMillan discloses an ultrasonic method and apparatus for determining the depth of cracks in pipe or other conduit.
U.S. Pat. No. 4,534,219 to Nadeau et al. discloses a device which relates the frequency of an acoustic wave to an indication of a crack within a test piece.